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A spectroscopic diagnostic system was designed to study the effects of different engine parameters on the chemiluminescence characteristic of HCCI combustion. The engine parameters studied in this work were intake temperature, fuel delivery method, fueling rate (load), air-fuel ratio, and the effect of partial fuel reforming due to intake charge preheating. At each data point, a set of time-resolved spectra were obtained along with the cylinder pressure and exhaust emissions data. It was determined that different engine parameters affect the ignition timing of HCCI combustion without altering the reaction pathways of the fuel after the combustion has started. The chemiluminescence spectra of HCCI combustion appear as several distinct peaks corresponding to emission from CHO, HCHO, CH, and OH superimposed on top of a CO-O continuum. A strong correlation was found between the chemiluminescence light intensity and the rate of heat release.

Experiments were performed on a homogeneously fueled compression ignition gasoline-type engine with a high degree of intake charge preheating. It was observed that fuels that contained lower end and/or non-branched hydrocarbons (gasoline and an 87 octane primary reference fuel (PRF) blend) exhibited sensitivity to thermal conditions in the surge tanks upstream of the intake valves. The window of intake charge temperatures, measured near the intake valve, that provided acceptable combustion was shifted to lower values when the upstream surge tank gas temperatures were elevated. The same behavior, however, was not observed while using isooctane as a fuel. Gas chromatograph mass spectrometer analysis of the intake charge revealed that oxygenated species were present with PRF 87, and the abundance of the oxygenated species appeared to increase with increasing surge tank gas temperatures. No significant oxygenated species were detected when running with isooctane.

Using spark retard during a cold-start is a very effective means of achieving fast catalyst light-off. In addition to obtaining faster catalyst light-off, retarding the spark also results in lower engine-out HC emissions. The objective of this research was to understand the reasons for the decrease in HC emissions with spark retard. In order to make the results as unambiguous as possible, the experiments were performed on a dynamometer at constant speed and load conditions using pre-vaporized, premixed gasoline. A zero-dimensional ring-pack crevice flow model was used to determine the mass flows into and out of the piston crevice during the engine cycle. The analysis showed that with spark retard a large fraction of the unburned fuel from the ring-pack re-entered the cylinder before the end of flame propagation, and was consumed by the flame when it extinguished on the cylinder wall.

A combination of engine experiments and modeling was used to investigate the effectiveness of adding di-tertiary butyl peroxide (DTBP) to gasoline to extend the light load limit in a homogeneous charge compression-ignition (HCCI) engine. The light load combustion stability limit at an engine speed of 1000 rev/min was reduced from a fueling rate of 9 mg/cycle with neat gasoline to 6.2 mg/cycle with 15% DTBP addition. A companion modeling study was performed using a three-zone, zero-dimensional engine model combined with detailed chemical kinetics. The fuel used in the model was composed of 85% iso-octane and 15% n-heptane. The model yielded trends which were similar to the experimental results. In particular, a linear relationship was found between the experimentally measured minimum fueling rate and the calculated location of maximum energy release rate for various levels of DTBP addition.

The homogeneous charge, compression-ignition (HCCI) combustion process has the potential to significantly reduce NOx and particulate emissions, while achieving high thermal efficiency and the capability of operating with a wide variety of fuels. This makes the HCCI engine an attractive technology that can ostensibly provide diesel-like fuel efficiency and very low emissions, which may allow emissions compliance to occur without relying on lean aftertreatment systems.

Homogeneous charge compression ignition offers the potential for significantly lower NOx emissions and up to a 20% improvement in fuel economy relative to a conventional port fuel injected spark ignition (SI) engine. The most significant challenge to developing a production viable HCCI engine is controlling the phasing of autoignition and the combustion rate across the speed and load range of the engine. This report describes an experimental and computational evaluation of controlling HCCI combustion at low loads by adding partial oxidation gas (POx), CO and H2, to the intake manifold. Experiments were performed using charge dilution obtained through conventional exhaust gas recirculation and by modified valve timings to increase the internal residuals. The experimental results showed that POx gas inhibited the low temperature energy release from n-heptane, but promoted the autoignition of isooctane.

The objective of this work was to understand the physics of combustion-generated pressure waves from Homogeneous Charge Compression Ignition combustion and the resulting audible noise that is produced. Experiments were performed with a single-cylinder engine operating in both SI and HCCI combustion modes, and comparisons were made between the pressure waves generation from the two types of combustion. Cylinder pressure oscillation amplitudes at the first circumferential mode frequency (5 to 6 kHz) generated in HCCI combustion are 5 to 10 times higher than those generated in SI knocking combustion without an undue increase in audible engine noise. Frequency analysis of the data showed that in knocking combustion a larger portion of the wave energy is contained within the higher order resonance modes. Cylinder block vibration measurements indicate that the cylinder liner significantly dissipates the wave energy below 8 kHz.